During the mobilization he was a cornet (= ensign) in the mounted artillery. Later, as a law student, he refused to sign the declaration of loyalty to the Germans and joined the resistance movement. He mapped activities of the Germans at Dutch airfields and passed them on to the allies.  He was arrested on December 8, 1943, suspected of conspiracy and illegal possession of weapons. After stays in various prisons, he was sentenced to eight years in penitentiary. Finally, he was sentenced to death by a special court-martial and subsequently executed.
There is no grave of Frank. Therefore, his father placed a memorial plaque for him at the grave of Frank’s mother. He wrote to the Oorlogsgravenstichting (War Graves Foundation): “My intention is to keep the memory of my son and of those days of cruel terror alive as long as possible among posterity and contemporaries.”  ♥
In the file of the Oorlogsgravenstichting in the National Archives, sheet 5 states the following:
Gerard Frank Smits was executed in Utrecht on April 4, 1944.
He had gone into hiding, among others, with Hagebout and van Goethem, a very good friend of his, on a farm in the Achterhoek region. Occasionally Frank ventured to Amsterdam and by betrayal in 1943, he fell into the hands of the SD. Upon arrest, a pistol is found on him. Prosecutor Schuhmacher, a hardcore Nazi, demands the death penalty; the German court martial sentences him in 1st instance to eight and then to 10 years in the penitentiary.
Schuhmacher, apparently rebuffed repeatedly with his demands, appeals and, through his party connections, succeeds in forming a new court-martial, with himself as chairman.
Previously imposed prison sentences are reopened and commuted to death sentences.
Schumacher disappears after the German surrender. Nazi investigator Simon Wiesenthal  eventually finds him in Limburg a/d Lahn as prosecutor; it will lead to his dismissal.
Sheet 8 in the same file  deals with the first OD law suit in Amersfoort and Maastricht, without any reference to a connection to Frank. He was also not convicted there and then, but in Utrecht and much later. Presumably it is an indication that he passed on his observations to the OD. Therefore he is listed here, with reservation, as belonging to the OD.
After his execution in Utrecht, he was cremated in Westerveld, according to his sister-in-law. 
His name is on one of the memorial stones in Fort de Bilt [7.1.] and the war memorial in Hulsberg [7.2.] In this village, moreover, the Frank Smitsstraat [7.3.] is named after him, not far from the family tomb on the church hill of St. Clemens Church, where his father had a commemorative plaque placed for him.